1. Field of the Invention
This invention relates to an air flow controlling apparatus for controlling a flow of air which is fed along an air course communicating, for example, with a boiler or the like.
Particularly, the present invention relates to an improvement in an air flow controlling apparatus for stabilizing an air flow when power supply to an electric motor for driving a ventilator which is to be controlled in ventilation is changed over between a commercial power source and a variable frequency power source.
2. Description of the Prior Art
Normally, control of an air flow is preferably with supply of power from a variable frequency power source from the point of view of saving of power. However, when a trouble occurs in such a variable frequency power source or when power which exceeds the capacity of a variable frequency power source (the capacity is limited in most cases to some low value from the point of view of a cost and so on) must be supplied to an electric motor, it becomes necessary to connect a commercial power source to the electric motor. Accordingly, it is sometimes necessary to effect changing over between a variable frequency power source and a commercial power source.
An air flow controlling apparatus of such a conventional type will be described below with reference to FIGS. 1 to 4 of the accompanying drawings.
FIG. 1 is a diagrammatic representation of a general construction of a conventional apparatus, partly shown in circuit diagram. In FIG. 1, reference numeral 1 designates a commercial power source (hereinafter referred to as "C power source"), 2 a variable frequency power source for receiving power from the C power source 1 and for supplying power having a converted frequency (hereinafter referred to as "V power source"), and 3 an electric motor which receives and is driven to rotate by power alternatively from the C power source 1 or the V power supply 2. The alternative supply of power is attained by selective opening and closing operations of a switch 4 interposed between the C power source 1 and the V power source 2, another switch 5 interposed between the V power source 2 and the electric motor 3, and a further switch 6 interposed between the C power source 1 and the electric motor 3. Reference numerals 4-1, 5-1 and 6-1 designate contacts which are opened and closed in response to opening and closing operations of the switches 4, 5 and 6, respectively. Further in FIG. 1, reference numeral 7 denotes an air course communicating from an entrance 7a to an exit 7b thereof to form an air course for a wind, a ventilator located in the air course 7 for receiving a rotational force of the electric motor 3 by way, for example, of a belt or the like to produce a wind, 9 a damper located in the air course 7 for adjusting resistance of the air course 7, and 10 an air course resistance controlling mechanism for controlling resistance in ventilating the air course 7. The air course resistance controlling mechanism 10 is connected to the damper 9 by means of a connecting bar 11 so as to adjust the opening of the damper 9 while the rotational frequency of the ventilator 8 is varied to control an air flow. Further in FIG. 1, reference numeral 12 designates a controlling value generator which produces a controlling value signal representative of the opening of the damper 9 and delivers it to the air course resistance controlling mechanism 10. The controlling value generator 12 includes a fixed value generating section 12a which provides a controlling value signal of a fixed value independently of an air flow of a load, and a variable value generating section 12b which provides a controlling value signal which is functional, for example, proportional to a load air flow. Reference numerals 13-1 and 13-2 designate controlling value change-over switches which are interlocked to each other to open and close, and to close and open, respectively, to alternatively couple controlling value signals produced from the fixed value generating section 12a and from the variable value generating section 12b to the air course resistance controlling mechanism 10. The change-over switch 13-1 is connected in series to the contacts 4-1, 5-1 between the fixed value generating section 12a and the air course resistance controlling mechanism 10 while the other change-over switch 13-2 is connected in parallel with the contact 6-1 between the variable value generating section 12b and the air course resistance controlling mechanism 10. Thus, the air course resistance controlling mechanism 10, the controlling value generator 12, and so on, constitute a damper controlling device.
FIG. 2 is a diagram showing relationships between the load air flow and the opening of the damper. In the diagram of FIG. 2, a straight line I illustrates the relationship where the damper opening is constant irrespective of the load air flow while another straight line II illustrates the relationship where the damper opening is in proportion to the load air flow. The fixed value generating section 12a produces a controlling value signal which varies in accordance with the straight line I while the variable value generating section 12b produces a controlling value signal which varies in accordance with the straight line II.
FIGS. 3 and 4 are diagrams showing characteristics upon changing over between the two power sources, FIG. 3 being a diagram upon changing over from the C power source to the V power source, and FIG. 4 being a diagram upon changing over from the V power source to the C power source. In both figures, curved lines IIIa and IIIb illustrate variations in the rotational frequency of the electric motor 3 relative to the time, curved lines IVa and IVb illustrate variations in the opening of the damper 9 relative to the time, and curved lines Va and Vb illustrate variations in the air flow relative to the time.
Now, operations of the conventional apparatus having the construction as described above will be described.
The electric motor 3 is driven from the V power source 2 to rotate the air blower 8 when the switches 4, 5 are in their respective closed positions while the switch 6 is in its open position. On the contrary, when the switches 4, 5 are in their open positions while the switch 6 is in its closed position, the electric motor 3 is driven from the C power source to rotate the air blower 8.
When the electric motor 3 is driven from the C power source 1, it is always rotated at a constant speed, and accordingly, a controlling operation for obtaining a predetermined load air flow is effected by adjustment of the opening of the damper 9. In particular, due to the fact that the contact 6-1 is closed, a controlling value signal is delivered from the variable value generating section 12b to the air course resistance controlling mechanism 10 so that the air course resistance controlling mechanism 10 operates in response to the controlling value signal to adjust the opening of the damper 9 by way of the connecting bar 11 so as to meet the relationship as represented by the straight line II of FIG. 2 to maintain the air flow to a predetermined level.
On the contrary, when the electric motor 3 is driven from the V power source 2, the speed of the motor 3 can be varied, and accordingly, a controlling operation for maintaining a predetermined air flow can be effected by controlling the rotational frequency of the air blower 8 which is driven by the electric motor 3. In this case, a controlling value signal is delivered from the fixed value generating section 12a to the air course resistance controlling mechanism 10 by way of the contacts 4-1 and 5-1 and the change-over switch 13-1 so that the air course resistance controlling mechanism 10 operates the damper 9, in response to the controlling value signal, so as to meet the relationship as represented by the straight line I of FIG. 2, that is, so as to maintain the air flow to a predetermined level.
It is to be noted here that, when an air flow is to be produced while the electric motor 3 is driven from the C power source 1, the opening of the damper 9 is reduced and hence causes a loss of power, but a parallel provision of two power sources is still employed as described hereinabove because there are some cases in which supply of power from the C power source 1 is necessitated, such as, upon occurrence of a trouble in the V power source 2, and so on, as described hereinabove.
Operations upon changing over between the two power sources will be described below.
At first, a changing over operation from the C power source 1 to the V power source 2 will be described. It is assumed that a C to V changing over switch not shown is thrown in at a time t.sub.1 as shown in FIG. 3. Then, immediately the switch 6 is opened and the switch 4 is closed. As a result, supply of power to the electric motor 3 is stopped and hence the rotational frequency of the electric motor 3 decreases gradually as indicated by the curved line IIIa of FIG. 3. In this instance, the damper 9 increases its opening gradually as indicated by the curved line IVa of FIG. 3 so as to maintain the air flow constant. Then, at a point of time t.sub.2 at which the rotational frequency of the electric motor 3 is lowered to a predetermined level determined by the power supplied from the V power source 2, the switch 5 is closed. Meanwhile, increase of the opening of the damper 9 is stopped when an opening is reached which is defined by the straight line I of FIG. 2 when the air flow is controlled by a rotational frequency controlling method in which power is supplied from the V power source. During the changing over operation, the air flow temporarily falls significantly below the predetermined level as indicated by the curved line Va of FIG. 3.
Now, a changing over operation from the V power source 2 to the C power source 1 will be described. In this case, before a V to C changing over switch not shown is thrown in, for example an operator manually operates repetitively to open and close the change-over switches 13-1, 13-2 to raise the speed of the electric motor 3 until a maximum rotational frequency is reached which is determined by power supplied from the V power source 2 while maintaining an air flow, and the opening of the damper 9 is also adjusted in accordance with the frequency: this is to prevent a shock or the like which may otherwise be caused by a large variation in rotational frequency upon changing over from the V power source 2 to the C power source 1 when the changing over is effected directly without any preparatory operation. Thus, at a point of time t.sub.3 (refer to FIG. 4) at which the rotational frequency of the electric motor 3 is in its stabilized condition after it has been raised to the maximum rotational frequency determined by the power supplied from the V power source 2, the V to C changing over switch not shown is at last thrown in. This immediately opens the switches 4, 5 and closes the switch 6 to connect the C power source 1 to the electric motor 3. Consequently, the rotational frequency of the electric motor 3 is increased gradually to a particular rotational frequency which is determined by the power supplied from the C power source 1 as shown by the curved line IIIb of FIG. 4, and after the particular rotational frequency has been reached, the rotation of the electric motor 3 is maintained in a stabilized condition. In this instance, the contact 6-1 is closed in response to closing of the switch 6 so that a signal from the variable value generating section 12b is coupled to the air course resistance controlling mechanism 10 to adjust the opening of the damper 9 to the load (refer to the straight line II of FIG. 2 and the curved line IVb of FIG. 4). During this changing over operation, the air flow temporarily increases high above the predetermined level as indicated by the curved line Vb of FIG. 4.
Thus, since the conventional air flow controlling apparatus has a construction which does not allow operations of the damper 9 to follow increasing and decreasing variations of the rotational frequency of the electric motor 3, that is, which does not allow harmonization of the opening and closing speed of the damper 9 with the increasing and decreasing variation of the rotational frequency of the electric motor 3 as described hereinabove, it is disadvantageous in that a large variation of the air flow is involved in a changing over operation of the apparatus. Moreover, as there is some time lag between the changing over operation of the C to V & V to C changing over switches and the damper start, the variation of the air flow becomes larger. In addition, when the air course 7 of the apparatus is connected, for example, to a boiler, the air flow to the boiler upon a changing over operation as described above is rendered unstable, resulting in instability of combustion in the boiler. Thus, it is a problem that a safe running of the boiler cannot be attained.